1. Technical Field
This invention relates generally to multiple access systems, and in particular to a wireless multiple access networks utilizing sets of orthogonal waveforms for separation of multiple simultaneously transmitting users.
Multiple access systems enable many simultaneous users to share the same fixed bandwidth radio spectrum. The bandwidth, which is allocated to any radio system, is always limited—for example, mobile phone systems use 25 MHz in each direction. On the other hand, we are interested in allowing multiple users simultaneous access to the network, for example in order to maintain multiple phone conversations in parallel. FDMA, TDMA and CDMA are the three major methods of sharing the available bandwidth to multiple users in wireless system. There are many extensions, and hybrid techniques for these methods, such as OFDM, and hybrid TDMA and FDMA systems.
AbbreviationsOFDMOthrogonal Frequency Division MultiplexingOFDMAOrthogonal Frequency Division Multiple AccessOSCMAOrthogonal Single Carrier Multiple AccessBPSKBinary Phase Shift KeyingQPSKQuaternary Phase Shift KeyingOQPSKOffset Quaternary Phase Shift KeyingPAMPulse Amplitude ModulationQAMQuadreature Amplitude ModulationOQAMOffset Quadrature Amplitude ModulationMSKMinimum Shift KeyingFSKFrequency Shift KeyingCPFSKContinuous-Phase Frequency Shift KeyingNCONumerically Controlled OscillatorFFTFast Fourier TransformIFFTInverse Fast Fourier TransformFECForward Error CorrectionISIInter-Symbol InterferenceICIInter-Carrier InterferencePAPRPeak-to-Average Power RatioMACMedium Access Control
2. Prior Art
In Frequency Division Multiple Access (FDMA), the available bandwidth is subdivided into a number of narrower band nonoverlapping channels. For each user is allocated a unique frequency band designated for data transmission. During a call, no other user can use the same frequency band. Each user is allocated a forward link channel (from the base station to the mobile phone) and a reverse channel (back to the base station), each being a single way link. The transmitted signal on each of the channels is continuous allowing analog transmissions. In TDMA the users send bursts of data in assigned time slots. This technique, while having numerous advantages, requires higher instantaneous transmit power or lower link loss due to higher instantaneous data rate.
There is a special interest in waveforms which maintain orthogonality under conditions of dispersion in the medium, and in particular in the wireless multipath channels. The Orthogonal Frequency Division Multiplex (OFDM) modulation, which uses a plurality of narrowband waveforms (subcarriers), was developed with this objective in mind. OFDM is similar to FDMA in the sense, that the available bandwidth is subdivided into multiple channels. Contrary to FDMA, however, OFDM achieves orthogonality between the subchannels in spite of the fact that their spectra overlap. This results in closer packing of frequency subchannel and more efficient use of the spectrum. Unlike conventional single-carrier modulation schemes—such as AM/FM (amplitude or frequency modulation)—that send only one signal at a time using one radio frequency, OFDM sends multiple high-speed signals concurrently on specially designed, orthogonal carrier frequencies. The result is much more efficient use of bandwidth as well as robust communications during noise and other interferences.
Recently an Orthogonal Frequency Division Multiple Access (OFDMA) technique was developed, which is a variant of OFDM. This technique assigns subsets of subcarriers to different transmitters in order to maintain orthogonality (sect separation) between the signals of different users. OFDMA facilitates adaptive bandwidth allocation to the users by varying the amount of subcarriers allocated to each user, and improves the Signal-to-Noise Ratio (SNR) to power-limited users by reducing their effective noise bandwidth. There are several strategies of allocating sets of subcarriers to users, optimizing different aspects of the system—multipath induced diversity, interference between different users, ease of channel estimation etc. The techniques for modulation and demodulation of OFDM waveforms using Fast Fourier Transform techniques is a common art today, as well as techniques for equalization and error correction decoding of OFDM in presence of multipath.
The relative disadvantage of OFDM is the high crest factor, known also as Peak-to-Average Power Ratio (PAPR) of the OFDM waveforms. A high peak to average ratio is created due to fact that at each instant the transmitted OFDM signal is a sum of a large number of slowly modulated subcarriers. A Single Carrier system avoids this effect, so the peak-to average transmitted power ratio for single carrier modulated signal is smaller.
This feature of OFDM motivated the proponents of Single Carrier modulation (which is better in PAPR respect) to develop processing techniques, which improve its performance in highly dispersive media.
Recently such a technique was developed, known as Frequency-Domain-Equalized Single Carrier modulation (FDE-SC). This technique utilizes concepts similar to OFDM in the sense that the receiver utilizes Fast Fourier Transform based processing for equalization. The FDE-SC waveforms, while lending themselves to convenient equalization, do not possess the qualities of OFDMA in the sense of adaptive bandwidth allocation in multiple access environments.
Therefore it is the object of the present invention to provide an innovative modification of the FDE-SC modulation, which enables using multiple-access setting in a way which allows both maintaining the orthogonality between the different received signals while also facilitating adaptive bandwidth allocation. Moreover, the proposed scheme allows creation of multiple access systems in which both Single Carrier (SC) signals and OFDM signals are used so that orthogonality is maintained for SC signals as well as to OFDM signals or any combination thereof. These advantages are achieved while maintaining the PAPR advantage of single carrier modulation. We shall denote the proposed scheme as Orthogonal Single Carrier Modulation (OSCM).
FIGS. 2a to 2d show basic examples of the transmitters structure according to prior art techniques. The structure of the transmitters is presented by the basic essential signal processing operations, for comparing the different prior art techniques and further to explain the improvements and modifications of the present invention.
FIG. 1a illustrates the basic structure of traditional single carrier transmitter. The operation of this transmitter is further explained. Block 200 describes a typical module for converting original data to signal symbols. The data received from data source 201 is first encoded using Forward Error Correction (FEC) encoder 202 (FEC encoding enables the receiver to correct errors automatically without requesting re-transmission). The encoded data bits are converted to carrier symbols by modulator 203, by converting small groups of bits to the required amplitude and phase based on pre-defined modulation scheme such as ASK, PSK, QAM, OQPSK, OQAM, MSK, CPFSK or any other appropriate method of mapping bit groups to symbols. As this set of operations is performed for all digital modulation schemes, we will collectively denote the process of converting the source data into a stream of encoded modulation symbols “data-to-symbol conversion” 200 and will not deal further with the internals of this process.
The shaping filter and the interpolator unit 204 define a signal shaping filtering which is required by the respective modulation schemes for achieving the desired spectral efficiency. For single carrier signals it is commonplace to use Square Root Raised Cosine (SRRC) characteristics, for achieving low Inter-Symbol Interference (ISI) after matched filtering on the receive side. The interpolation filter increases the sampling rate and rejects the extra images of the signal spectrum resulting from the interpolation operations.
Block 208 aggregates several components jointly to convert the signal samples into the actually transmitted signal. The signal is initially converted from digital samples to an analog signal using a Digital-to-Analog converter 205. The analog signal is further filtered by an analog filter 206, removing any unwanted frequency components which remained after the action of the interpolator within block 104. At the end, an up-converter 207 converts the signal to the desired frequency band and transduces the resulting signal into the transmission medium, for example an antenna in the case of wireless transmission. As the operations contained within block 208 are commonplace in every digital transmitter, we will refer to those collectively as “Samples-to-signal conversion” (block 208).
FIG. 2b illustrates the structure of FDE-SC transmitter. After data-to-symbol conversion 211 (same as block 100 in FIG. 1a), the symbol stream is segmented by unit 212 creating blocks of fixed size. Each block of symbols is processed by the cyclic prefix generator 213, creating a waveform with periodic property which is essential to the proper functioning of the fast Fourier transform (OFT) operation at the receiver end. The resulting stream of symbols is then subjected to filtering and interpolation (214) and to conversion to the actually transmitted signal in block 215.
The process of OFDM transmitter bears similarity to the FDE-SC transmitter as seen in the illustration of the transmitter structure (FIG. 2c). In this process the generated symbols are segmented into groups (222), each corresponding to a separate OFDM symbol. The symbols are located within a numeric array representing frequency samples, the numeric array is further edited by inserting zeros and pilot symbols at appropriate locations using unit 323, and then converted from frequency into time samples by inverse FFT modulation unit 224. The time samples are further processed by cyclic prefix generator 225, and converted to the actually transmitted signal in block 226.
The OFDMA transmitter (FIG. 2d) is similar to the OFDM transmitter in terms of processing. The main difference is that the transmitter utilizes only a fraction of the frequency subcarriers within the operational frequency channel. The rest of the subcarriers are used by other transmitters within the multiple-access network, and the sets of subcarriers are assigned to each transmitting station according to allocation policy of the Medium Access Control (MAC) entity. In order to transit on the assigned subcarriers only, block 233 inserts the encoded data symbols to the corresponding locations within the numeric array, adds pilot subcarriers and fills with zeros all the unused locations. From here the processing continues as in OFDM transmitter, by adding a cyclic prefix 235 and converting the samples to the actually transmitted signal in block 236.
At this point, it is appropriate to discuss the subcarrier allocation strategies used in different OFDMA systems, as these considerations are applicable to the transmitters operating according to present invention. The dominant factors affecting the strategy are the correlation in channel coefficients of adjacent subcarriers, on one hand, and the interference between adjacent subcarriers (ICI) on the other hand. One possible policy is to pick the allocated subcarriers from locations spread all over the frequency channel. An alternative policy is to cluster the subcarriers assigned to a transmitter together in order to optimize the interaction between signals of different stations (as only the edges of the allocations interact). This policy also has beneficial effect on the estimation of channel response. Within the wideband subcarrier allocation policies, we may differentiate between those, which assign the subcarriers to a station at regular intervals, and those, which allocate irregularly spaced locations. The irregular allocation of OFDMA subcarriers has the beneficial effect that for any two stations only a fraction of the subcarriers are adjacent, thus reducing the mutual interference. The irregular OFDMA technique was adopted in DVB-RCT uplink channel and in 802.16a broadband wireless access standard OFDMA mode.